Manual wheelchairs, such as the Quickie XTR, Quickie 2, Invacare Xtra and E&J Epic wheelchairs, are typically equipped with two main side wheels and two front caster wheels. The four wheels on most wheelchairs are arranged so that the seat of the wheelchair and the bottoms of the wheels remain even and level when the wheels are in contact with a level, even or flat ground surface. Such an arrangement works well when the wheelchair travels over flat ground and generally provides smooth transportation for a wheelchair occupant. However, when such a wheelchair travels over a curb, e.g., at the edge of a sidewalk onto a street, or otherwise moves from an upper surface to a lower surface, the wheel arrangement can cause the wheelchair to tip when the bottoms of the front caster wheels drop over the curb into a position that is at a different elevation, e.g., lower, than the level of the bottom of the main side wheels. Such tipping can present a potential danger to a wheelchair occupant and can cause accidents which may result in spinal or other injuries to a wheelchair occupant if the chair quickly drops down off the curb.
Improvements to manual wheelchairs have been previously proposed for ascending and descending curbs or other uneven surfaces, and navigating obstacles. One example is the device described in U.S. Pat. No. 4,962,942. Another example is a power wheelchair for traversing uneven terrain, such as the wheelchair described in U.S. Pat. No. 6,554,086. However, those wheelchairs require electrical or computerized components or input from the wheelchair occupant.